In the majority of applications printing proceeds by pressure contact of an ink-loaden printing form with an ink-receiving material which is usually plain paper. The most frequently used impact printing technique is known as lithographic printing based on the selective acceptance of oleophilic ink on a suitable receptor.
In recent times however so-called non-impact printing systems have replaced classical pressure-contact printing to some extent for specific applications. A survey is given e.g. in the book "Principles of Non Impact Printing" by Jerome L. Johnson (1986), Palatino Press, Irvine, Calif. 92715, USA.
Among non-impact printing techniques ink jet printing has become a popular technique because of its simplicity, convenience and low cost. Especially in those instances where a limited edition of the printed matter is needed ink jet printing has become a technology of choice. A recent survey on progress and trends in ink jet printing technology is given by Hue P. Le in Journal of Imaging Science and Technology Vol. 42 (1), January/February 1998, which is hereby included as reference.
In ink jet printing tiny drops of ink fluid are projected directly onto an ink receptor surface without physical contact between the printing device and the receptor. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving the print head across the paper or vice versa. Early patents on ink jet printers include U.S. Pat. Nos. 3,739,393, 3,805,273 and 3,891,121.
The jetting of the ink droplets can be performed in several different ways. In a first type of process a continuous droplet stream is created by applying a pressure wave pattern. This process is known as continuous ink jet printing. In a first embodiment the droplet stream is divided into droplets that are electrostatically charged, deflected and recollected, and into droplets that remain uncharged, continue their way undeflected, and form the image. Alternatively, the charged deflected stream forms the image and the uncharged undeflected jet is recollected. In this variant of continuous ink jet printing several jets are deflected to a different degree and thus record the image (multideflection system).
According to a second process the ink droplets can be created "on demand" ("DOD" or "drop on demand" method) whereby the printing device ejects the droplets only when they are used in imaging on a receiver thereby avoiding the complexity of drop charging, deflection hardware, and ink recollection. In drop-on-demand the ink droplet can be formed by means of a pressure wave created by the mechanical motion of a piezoelectric transducer (so-called "piezo method"), or by means of discrete thermal pushes (so-called "bubble jet" method, or "thermal jet" method).
Ink compositions for ink jet typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such an ink is dependent on the ink jetting method used and on the nature of the substrate to be printed. The ink compositions can be roughly divided in:
water based; the drying mechanism involves absorption, penetration and evaporation; PA1 oil based; the drying involves absorption and penetration; PA1 solvent based; the drying mechanism involves primarely evaporation; PA1 hot melt or phase change: the ink vehicle is liquid at the ejection temperature but solid at room temperature; drying is replaced by solidification; PA1 UV-curable; drying is replaced by polymerization.
It will be readily understood that the first two types of ink compositions require a receiving medium that is more or less absorptive. On the contrary, for non-absorbent substrates solvent based inks, hot melt inks or UV-curable inks will be better suited.
Early patents on water-based inks include U.S. Pat. Nos. 3,903,034, 3,889,269, 3,870,528, 3,846,141, 3,776,742 and 3,705,043. However, it was recognized early that systems based on water-based inks suffer from a number of disadvantages such as: (a) they require water evaporation and therefore an extensive drying system, especially when printing speed is important; (b) large printed areas tend to cockle, (c) the images are sensitive to wet and dry rubbing, (d) inks of low viscosity tend to tip dry on the orifice which can be avoided by the use of humectants, usually glycols, which then increase viscosity. The use of polar solvent based inks can overcome some of the problems inherent to water-based inks, but in its turn causes other problems such as the possible generation of toxic or inflammable vapours. Therefore efforts were conducted to the development of low-solvent ink compositions. In this research the concept of UV-curable ink compositions was generated, of which a survey is given hereinafter.
An important basic patent on ink compositions for ink jet, satisfying the need for a low solvent content, and containing a UV-curable compound is U.S. Pat. No. 4,303,924. It describes an ink jet printing process using charged droplets wherein the ink composition contains (a) a multifunctional unsaturated UV-curable compound, (b) a monofunctional unsaturated compound, (c) a reactive synergist, (d) a colorant, (e) an oil soluble salt for conductivity, (f) a photoinitiator, and (g) an organic polar solvent, preferably in a small amount. Several examples of monomers containing acrylate, epoxy, and vinyl functional groups are disclosed.
In EP 0 071 345 a jet ink composition is claimed comprising (A) a cationically polymerizable epoxy resin chosen from particular classes, (B) a photoinitiator, (C) a colorant, (D) a blend of organic solvents.
In U.S. Pat. No. 4,680,368 a UV-curable ink, not limited to ink jet, is disclosed comprising (A) a poly(urethane-(meth)acrylate), (B) a radically polymerizable compound and (C) a photoinitiator.
According to U.S. Pat. No. 4,978,969 the ink composition comprises 12-80% of a UV curable adhesive, 3-10% of a pigment, and 10-40% of a solvent.
In EP 0 456 039 B1 an ink composition for ink jet is disclosed that is free of volatile organic solvent and contains a colorant, a polar conductive compound, and one or more monomers. In the analogous EP 0 540 203 B1 a non-conductive ink composition, free of volatile solvent, is disclosed, said composition again comprising one or more monomers and a colorant.
In U.S. Pat. No. 5,270,368 the ink composition contains at least two acrylate types, being an aromatic acrylate with carboxyl groups, and an epoxy acrylate.
According to EP 0 658 607 an aqueous ink contains a pigment, a water-soluble resin for dispersing the pigment, a water-soluble UV-curable monomer and a photoinitiator.
In U.S. Pat. No. 5,623,001 an ink is described comprising (a) 20-75% water, (b) a water-mixable UV-curable compound, preferably an acrylate oligomer, (c) a photoinitiator and (d) a colorant.
According to U.S. Pat. No. 5,641,346 the ink jet ink contains a colorant, a liquid phase comprising water, and an epoxy compound and/or a vinyl ether compound.
In WO 97/31071 a radiation-curable ink jet composition is described comprising from 80% to 95% of a polyfunctional (poly)alkoxylated acrylate monomer.
Summarizing, a radiation curable ink composition may in general contain one or more radiation curable prepolymers, or oligomers, radiation curable monomers or reactive diluents, optionally one or more photoinitiators, colorants, and other additives.
Many classes of radiation curable compounds are not suitable in radiation curable compositions for ink jet printing due to their high viscosities. On the other hand, using other classes of radiation curable compounds with lower viscosities results in a lower polymerization speed.
The present invention extends the teachings on radiation curable ink compositions for ink jet.